Construction and evaluation of self‐cloning bottom‐fermenting yeast with high SSU1 expression

Aim: To construct a self‐cloning brewer’s yeast that can minimize the unfavourable flavours caused by oxidation and certain kinds of sulfur compounds. Methods and Results: DNA fragments of a high‐expression promoter from the TDH3 gene originating from Saccharomyces cerevisiae were integrated into the promoter regions of the S. cerevisiae‐type and Saccharomyces bayanus‐type SSU1 genes of bottom‐fermenting brewer’s yeast. PCR and sequencing confirmed the TDH3 promoter was correctly introduced into the SSU1 regions of the constructed yeasts, and no foreign DNA sequences were found. Using the constructed yeasts, the concentration of sulfite in fermenting wort was higher when compared with the parent strain. In addition, the concentrations of hydrogen sulfide, 3‐methyl‐2‐buten‐1‐thiol (MBT) and 2‐mercapto‐3‐methyl‐1‐butanol (2M3MB) were lower when compared with the parent strain. Conclusion: We successfully constructed a self‐cloning brewer’s yeast with high SSU1 expression that enhanced the sulfite‐excreting ability and diminished the production ability of hydrogen sulfide, MBT and 2M3MB. Significance and Impact of the Study: The self‐cloning brewer’s yeast with high SSU1 expression would contribute to the production of superior quality beer with a high concentration of sulfite and low concentrations of hydrogen sulfide, MBT and 2M3MB.     

Characterization of maltotriose transporters from the Saccharomyces eubayanus subgenome of the hybrid Saccharomyces pastorianus lager brewing yeast strain Weihenstephan 34/70

The genome from the Saccharomyces pastorianus industrial lager brewing strain Weihenstephan 34/70, a natural Saccharomyces cerevisiae/Saccharomyces eubayanus hybrid, indicated the presence of two different maltotriose transporter genes: a new gene in the S. eubayanus subgenome with 81% of homology to the AGT1 permease from S. cerevisiae, and an amplification of the S. eubayanus MTY1 maltotriose permease previously identified in S. pastorianus yeasts. To characterize these S. eubayanus transporter genes, we used a S. cerevisiae strain deleted in the AGT1 permease and introduced the desired permease gene(s) into this locus through homologous recombination. Our results indicate that both the MTY1 and AGT1 genes from the S. eubayanus subgenome encode functional maltotriose transporters that allow fermentation of this sugar by yeast cells, despite their apparent differences in the kinetics of maltotriose‐H⁺ symport activity. The presence of two maltotriose transporters in the S. eubayanus subgenome not only highlights the importance of sugar transport for efficient maltotriose utilization by industrial yeasts, but these new genes can be used in breeding and/or selection programs aimed at increasing yeast fitness for the efficient fermentation of brewer's wort.     

白蚁肠道共生微生物的作用:酵母,一个“隐身”的特别菌群

长期以来,白蚁对木质纤维素的降解能力令人惊叹,毫无疑问,其在全球碳循环中扮演着一个十分重要的角色。这一强大功能的实现极大地依赖于一种特别的肠道\"消化液(digestome)\",它的构成不仅包括了来自白蚁自身产生的木质纤维素降解酶系统,还来源于独特与多样的肠道共生微生物的贡献(包括了古细菌、细菌、酵母以及其他真核生物),它们的协同作用能有效地将木质纤维素生物质高效转化为乙酸、甲烷、二氧化碳、氢气等物质。然而,到目前为止,我们对这类昆虫的独特肠道生物转化系统的认识还很不深入,特别是针对肠道内的那些各类共生微生物菌群的功能、白蚁与共生微生物间的相互关系、以及潜在的科学与应用价值还无法给予明确的科学解释,更不用说针对其肠道中的共生酵母菌群,一类通常被忽略的独特微生物。近20多年来,越来越多的研究证据表明,白蚁肠道共生酵母在与寄主的关系中表现了不可或缺的重要性与独特功能,已被证明广泛分布于不同白蚁及许多其他昆虫的肠道中。随着近20年来越来越多昆虫肠道共生微生物酵母群被发现和鉴定,他们潜在的功能以及与寄主的共生机制被逐步解析,这些研究结果进一步揭示了\"隐身\"的昆虫肠道酵母类微生物菌群与寄主的营养、关键生物质转化过程中的重要酶系统、转化过程中的关键中间产物的转化与利用、抵御外源性的重要病原物,甚至对白蚁种群繁衍的远缘交配等方面均可能发挥了重要和不可缺少的作用。本文将试图归纳相关研究的最新进展,系统总结与解析白蚁肠道来源共生酵母的重要科学价值及其在不同领域的潜在应用前景。     

Physiological and genetic stability of hybrids of industrial wine yeasts Saccharomyces sensu stricto complex

Aims: The aim of this study was to examine the physiological and genetic stability of hybrids of industrial wine yeasts Saccharomyces sensu stricto complex subjected to acidic stress during fermentation. Methods and Results: Laboratory‐constructed yeast hybrids, one intraspecific Saccharomyces cerevisiae × S. cerevisiae and three interspecific S. cerevisiae ×Saccharomyces bayanus, were subcultured in aerobic or anaerobic conditions in media with or without l ‐malic acid. Changes in the biochemical profiles, karyotypes and mitochondrial DNA profiles of the segregates were assessed after 50–190 generations. All yeast segregates showed a tendency to increase the range of the tested compounds utilized as sole carbon sources. Interspecific hybrids were alloaneuploid and their genomes tended to undergo extensive rearrangement especially during fermentation. The karyotypes of segregates lost up to four and appearance up to five bands were recorded. The changes in their mtDNA patterns were even broader reaching 12 missing and six additional bands. These hybrids acquired the ability to sporulate and significantly changed their biochemical profiles. The alloaneuploid intraspecific S. cerevisiae hybrid was characterized by high genetic stability despite the phenotypic changes. l ‐malic acid was not found to affect the extent of genomic changes of the hybrids, which suggests that their demalication ability is combined with resistance to acidic stress. Conclusions: The results reveal the plasticity and extent of changes of chromosomal and mitochondrial DNA of interspecific hybrids of industrial wine yeast especially under anaerobiosis. They imply that karyotyping and restriction analysis of mitochondrial DNA make it possible to quickly assess the genetic stability of genetically modified industrial wine yeasts but may not be applied as the only method for their identification and discrimination. Significance and Impact of the Study: Laboratory‐constructed interspecific hybrids of industrial strains may provide a model for studying the adaptive evolution of wine yeasts under fermentative stress.     

Screening of yeast strains for phytase activity

A screening method was developed to elucidate the ability of different yeast strains to utilize phytic acid as sole phosphorus source. The growth test in liquid culture in a microtiter plate with phytic acid as sole phosphorus source was shown to be a reliable, fast and easy-to-use screening method. We tested 122 strains from 61 species with our method and observed growth differences among species and strains that were not detectable on solid medium. Specific phytase activities were measured for 10 yeasts strains, selected due to their strong growth in the liquid medium. Strains of Arxula adeninivorans and Pichia anomala reached the highest volumetric phytase activities. Arxula adeninivorans also displayed the highest intra- and extracellular specific activities. There were large differences in both extra- and intracellular phytase activities among species. Strain-specific extracellular phytase activities were detected in P. anomala . The presence of free phosphate in the media completely suppressed the extracellular phytase activity and also reduced intracellular phytase activity for all tested yeast strains.     

Possible use of Biolog methodology for monitoring yeast presence in alcoholic fermentation for wine‐making

Aims: A research was undertaken to explore the possibility to use Biolog system of microbial metabolic characterization for the monitoring of yeast population evolution during alcoholic fermentation for wine production. Methods and Results: An application of Biolog system was employed for the characterization of yeasts of oenological interest, in pure cultures and mixed consortia, in various cell concentrations. The system’s capacity to discriminate among different cell concentrations of the same yeast strain was ascertained, along with the capacity to discriminate between mixed and pure populations. Conclusions: The tested application of Biolog system resulted suitable for a quick recognition (24 h) of the presence of starter cultures within mixed populations of autochthonous yeasts. Such discrimination was confirmed with the one resulting from molecular techniques. Significance and Impact of the Study: The study suggests the possibility to employ Biolog system for an early monitoring of yeast evolution in modern wine‐making fermentations, where specialized yeasts are more and more frequently used as starters and their ability to overcome autochthonous yeast populations is crucial.     

Modelling Biolog profiles' evolution for yeast growth monitoring in alcoholic fermentation

Aims: A research was undertaken to explore the possibility to express with suitable mathematical models Biolog metabolic curves obtained for oenological yeasts and to use such models for monitoring yeast growth in alcoholic fermentation. Methods and Results: Experimental curves of metabolic activity in Biolog YT microplates, obtained in a previous work for various oenological yeast strains in pure cultures and mixed populations, at various cell concentrations, have been modelled with Gompertz’s, Gompertz’s modified and Lindstrom’s mathematical equations. Lindstrom’s model proved to be the most suitable to fit the curves of the oenological yeasts under study, providing the highest correlation coefficients between experimental and calculated data. The model made it possible to recognize, in mixed yeast populations, the presence of active dry yeasts used for guided fermentations. Model’s constant parameters were used for a numerical characterization of yeast curves. Conclusions: The application of the model to the experimental data resulted to be suitable for an early prediction of the successive evolution of yeast growth. Significance and Impact of the Study: The results obtained indicate the possibility to develop protocols for monitoring yeast presence during alcoholic fermentation, with an early assessment of the correct evolution of their growth, especially when active dry yeasts are employed.     

Food insecurity: Prevalence,causes, and the potential of transgenic `Golden Rice'

This paper reviews a number of recent social science publications on the nature and causes of food insecurity in low and middle-income countries. The focus is on one specific element of food insecurity, vitamin A deficiency, which is widespread in developing countries and causes blindness and early death among millions of children. A new approach in the fight against vitamin A deficiency is `Golden Rice'. The paper explores the pros and cons of this recently developed transgenic rice variety, and tries to answer the question whether in this particular case, genetic modification technology provides a solution to food insecurity. It is concluded that Golden Rice is one of the options available, but not necessarily the most effective one.     

酶的固定化技术最新研究进展

酶是一种高效、绿色、应用广泛的生物催化剂,因其固定化形态在多种性质上均优于游离态,酶固定化技术应运而生并不断发展。我国固定化技术研究始于20世纪70年代,目前固定化酶在食品、医疗、能源、环境治理等领域得到了广泛的应用,但现有固定化技术仍存在适用范围小、成本较高等缺陷。因此,在较为成熟的传统固定化技术基础上,研究者们对新型固定化技术的研究与创新进行了大量尝试,形成了一批以固定化载体和固定化方式为核心的新型固定化技术。文中作者结合团队十余年对固定化技术的研究和理解,归纳介绍了新型酶固定化技术的发展方向和应用趋势,并阐述了对固定化技术未来发展的理解和建议。